1. Field of the Invention
The present invention relates to a high strength bolt excellent in delayed fracture resistance and a method of production of the same, more particularly relates to a high strength bolt excellent in delayed fracture resistance having a 1400 MPa class or higher tensile strength and a method of production of the same.
2. Description of Related Art
The high strength bolts used in the automobile, general machinery, bridge, civil engineering, and other various industrial fields are made from steel such as the chromium steel (SCr) and chrome molybdenum steel (SCM) defined in the JIS G 4104 and JIS G 4105 with C concentrations, by mass %, of 0.20 to 0.35%, that is, medium carbon steels, which are quenched and tempered. However, it is known that when the above-mentioned steel materials exceed 1300 MPa in tensile strength, the risk of hydrogen embrittlement, in particular the risk of a delayed fracture phenomenon due to the hydrogen entering it from its environment during use, rises. For this reason, for example, in the case of use for construction, the use of SCr steel and SCM steel is presently limited to steel for a tensile strength of the 1150 MPa class. To enable safe use of material with a higher strength, the hydrogen embrittlement resistance as represented by the delayed fracture resistance has to be improved.
As a conventional discovery for improvement of the delayed fracture resistance of high strength steel, for example, Japanese Patent Publication (B2) No. 3-243744 proposes to make the pre-austenite grains in the microstructure finer, to transform the microstructure to a bainite structure, and otherwise control the microstructure.
Art for making the pre-austenite grains finer is proposed, in addition to the above-mentioned prior art, in Japanese Patent Publication (B2) No. 61-64815, Japanese Patent Publication (B2) No. 64-4566, and Japanese Patent Publication (B2) No. 3-243745 as well, but in each case, no great improvement in the delayed fracture resistance is achieved—as pointed out already in Japanese Patent Publication (A) No. 2000-26934.
Further, the method of transforming the microstructure to a bainite structure has a recognizable effect of improving the delayed fracture resistance, but the rise in the manufacturing costs required for transformation to a bainite structure becomes an issue. In addition, when transforming the microstructure to a bainite structure, in general the strength tends to drop compared with the case of a martensite structure. In particular, to secure a high strength such as the tensile strength of 1400 MPa or more shown in the present invention, further addition of alloying chemical ingredients and other measures are essential and therefore invite an increase in the manufacturing costs and result in less of an economic effect.
On the other hand, the above-mentioned Japanese Patent Publication (A) No. 2000-26934 proposes to diffuse and distribute single or composite precipitates of oxides, carbides, and nitrides into the steel for trapping the hydrogen and increase the amount of critical hydrogen causing delayed fracture (below, indicated as the “critical diffusable hydrogen amount”) so as to improve the delayed fracture resistance. In this invention, as one mechanism for improving the delayed fracture resistance, the method of using carbides formed by quenching and tempering is mentioned. To improve this delayed fracture resistance, limitation of the chemical ingredients and the quenching, tempering, and other heat treatment conditions for the optimal diffusion and distribution of single or composite precipitates of oxides, carbides, and nitrides for trapping the hydrogen is essential.